I don't think SGI thought it was worthwhile to support Windows NT on their MIPS hardware. I suspect that SGI knew at the time that Pentium Pro and Pentium II+ were going to hand MIPS its butt, and I think part of building the 320/540 was that SGI wanted to move their customers over to platforms that would be easier to support and better for the needs.
Remember, SGI wasn't really in the business of selling generic workstations and servers for chores. They were selling appliances for HPC and ultra high end graphics work, including video, 3d rendering, high end CAD, etc.
Unfortunately, the software developers didn't move over to NT-on-SGI and NT didn't really gain the ability to compete with SGI's highest end hardware (the Octane could use 8 gigs of RAM, for example) until well after MIPS had stopped being viable. SGI tried later to migrate their customers to Linux on Itanium, but I don't think they were having it then, either.
The last of the MIPS systems were bought and sold not because they were actually better than anything else at what they were doing, but on pure inertia.
Anyway MIPS sold chips to a few other companies (including, at least, NEC, I'm sure there were others) and they built NT-compatible MIPS systems.
Alpha would have been a super interesting target for NT4. As I understand, it's the basis for the 64-bit support Windows XP eventually got, so it would have been neat to see DEC/Compaq/HP put that in earlier to support their high end hardware well before Windows NT eventually got a 64-bit release in ~2003.
It would be interesting to see NT4 on Apple's PPC hardware, but I don't think it would end up being that special, mainly because there would be no software for it, and I don't know off hand if the PPC version of NT4 ever ended up with the x86 emulation bits. If they existed on PPC, they were certainly better on Alpha, where there was win32 support and where the fastest alphas at the time were competitive with Pentiums at PPC performance using that emulator. (Which is to say: Imagine a world where NT4/Alpha existed long enough to get more native software. Especially stuff that NT4 had and that Macs were used for frequently at the time.)